Nonlinear Frequency Conversion (NLFC) is a widely used optical technique for generating specific wavelengths using laser devices. In an NLFC device, light with a fundamental wavelength enters an NLFC component which converts some or all of the light with the fundamental wavelength into light with a converted wavelength. A common variation of this technique uses light with fundamental wavelength that is frequency-doubled, resulting in a converted wavelength which is half the fundamental wavelength, a process known as second harmonic generation (SHG). The NLFC process does not convert all of the light with the fundamental wavelength, leading to a spatial overlap of the light with the converted wavelength and light with the fundamental wavelength exiting the NLFC component.
Many applications for NLFC devices require only the light with the converted wavelength, so some or all of the light with the fundamental wavelength exiting the NLFC component must be removed. This may be achieved by the spatial separation of the two beams (fundamental and converted) and then absorption of the fundamental beam.
Frequency-doubling devices can be categorised depending on the polarisation properties of the fundamental and converted beam. In “type I” SHG the linearly polarised converted beam exiting the NLFC device has an orthogonal polarisation relative to the linearly polarised fundamental beam. The 90° change in polarisation can be exploited to separate the fundamental and converted beams by using Brewster mirror reflection, as taught in U.S. Pat. No. 8,559,471 (Mao, issued Oct. 15, 2013). A mirror which has high reflectivity to the converted beam and transmits the majority of the fundamental beam is oriented at the Brewster angle in a device described by Tangtrongbenchasil et al. [Japanese Journal of Applied Physics 47, 2137, (2008)].
Absorption of laser light may be achieved with a beam absorber—a cavity designed to trap the light—into which the laser light is directed. Some laser light may be reflected or scattered on contact with a beam absorber and can escape the beam absorber. Examples of beam absorbers designed to reduce this escaping scattered light are found, for example U.S. Pat. No. 8,047,663 (Pang et al., issued Nov. 1, 2011), where the beam absorber is fashioned as a tapered spiral terminating in an absorption chamber. However, such designs have the disadvantage of being comparatively difficult to manufacture and bulky compared to simpler cavities.
A combined wavelength rejection mirror and beam absorber is illustrated in Japanese Pat. App. No. 2005337715A (Toshiyasu et al., published Dec. 8, 2005), which includes wavelength rejection mirrors in a casing which may be configured to be absorbing.